Trevor McDonald has spoken of his shock while interviewing members of the mafia for his latest documentary.

The veteran news broadcaster rubbed shoulders with some of history's most 'successful' mafia men for the film and admits he was stunned by how honest they were about the people they had killed.

Talking on This Morning with Phillip Schofield and Amanda Holden, Trevor said: "I was shocked by the brutality of what they did and the frankness with which they talked about that brutality.

This Morning - Trevor McDonald (Image: ITV)

"Strangely, they were quite prepared to talk. I spoke to one guy and said, 'You killed a number of people, do you sleep at night?' He said, 'I sleep terribly well'."

Trevor then asked him if he kept count of the amount of people he killed, to which the man responded: "No that would be sick. They were out to get me, so I killed them first."

In the first episode, Sir Trevor meets John Alite, who was an enforcer for John Gotti Senior, the notorious New York Mafia godfather, during the 1980s and 1990s.

For the first time Trevor takes viewers on an immersive journey into the American Mafia (Image: ITV)

Trevor also encounters Michael DiLeonardo – better known as Mikey Scars – who fears he'll be executed after testifying against members of the Gambino crime family in court, leading to 80 successful convictions.

The former newsreader admits he was surprised the men were so open about the criminal underworld but enjoyed getting to know more about the American mafia.

He said: "I would not have done it myself, but I'm very glad they did, I think they've contributed to what is a fascinating look at their lives."

The Mafia With Trevor McDonald is a two-part documentary starting on ITV this evening.